Watermark embedding is an important aspect of copy protection strategies. Although most copy protection schemes deal with protection of electronically distributed contents (broadcasts, storage media), copy protection is also desired for movies shown in theaters. Nowadays, illegal copying of cinema material by means of a handheld video camera is already common practice. Although the quality is usually low, the economical impact of illegal VHS tapes, CD-Videos and DVDs can be enormous. For this reason, cinema owners are obliged to prevent the presence of video cameras on their premises. Not following this rule may be sanctioned with a ban on the future availability of content. In view hereof, it is envisioned to provide that a watermark will be added during show time. The watermark is to identify the cinema, the presentation time, operator, etc.
Robustness to geometric distortions is a key requirement for such watermark embedding schemes. A handheld camera will not only seriously degrade the video by filtering (the optical path from the screen to the camera, transfer to tape, etc.) but also seriously geometrically distort the video (shifting, scaling, rotation, shearing, changes in perspective, etc.). In addition, these geometrical distortions can change from frame to frame.
A prior-art method of embedding a watermark in cinema movies, which meets the robustness requirements, is disclosed in Jaap Haitsma and Ton Kalker: A Watermarking Scheme for Digital Cinema; Proceedings ICIP, Vol. 2, 2001, pp. 487-489. The robustness to geometric distortions is achieved by exploiting only the temporal axis to embed the watermark. The watermark is a periodic pseudo-random sequence of watermark samples having two distinct values, e.g. ‘1’ and ‘−1’. One watermark sample is embedded in each image. The value ‘1’ is embedded in an image by increasing a global property (e.g. the mean luminance) of the image, the value ‘−1’ is embedded by decreasing said global property.
The prior-art watermark embedding method actually embeds flicker. By embedding the same watermark sample in a number of consecutive images, the flicker is made imperceptible (the human eye is less sensitive to low-frequency flicker).
Flicker of the recorded movie is also caused by a) the typical mismatch between the cinema projector's frame rate (24 frames per second) and the camcorder's frame rate (25 fps for PAL, 29.97 fps for NTSC), and b) the difference between the two display scan formats (progressive vs. interlace). This kind of flicker is so annoying that de-flickering tools have been made widely available to the public. For example, a de-flicker plug-in for the video capturing and processing application “Virtualdub” has been found on the Internet.
A problem of the prior-art watermark embedding scheme is that de-flicker tools also remove the embedded watermark.